1. Field of the Invention
The present invention relates to optical element units used in exposure processes, in particular to optical element units of microlithography systems. It further relates to optical exposure apparatuses comprising such optical element units. Furthermore, it relates to methods of manufacturing such optical element units. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
2. Description of the Related Art
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. Such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semi-conductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system.
Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure is process and over the lifetime of the system. As a consequence, the optical elements of such an optical system must be supported in a defined manner in order to maintain a predetermined spatial relationship between the optical elements to provide a high quality exposure process.
Depending on the wavelength of the light used in such exposure processes, it is often necessary to maintain a gas atmosphere within the respective optical element units in order to reduce absorption effects. During the exposure process pressure differences between the gas atmosphere within the optical element unit and the environment surrounding the optical element unit may occur. These pressure differences may cause a position variation of the ultimate optical element, typically a so called last lens element, located near or at the exit end of the optical element unit and typically separating the interior of the optical element unit from the surrounding environment.
Depending on the design of the respective last lens element, in particular on the optical sensitivity of the last lens element to position variations, such position variations may have a considerable adverse effect on the imaging accuracy and, thus, on the overall quality of the exposure process. To largely avoid these effects, currently, considerable effort is necessary to hold such an ultimate optical element in a manner which is as rigid as possible. This is particularly complicated if, for other purposes, the ultimate optical element has to be held in an adjustable manner.